Microfabricated, stress-engineered springs can be used in applications ranging from electrical interconnects, to high density probe cards to on-chip high quality factor out-of-plane inductors. The process for manufacturing these springs may include depositing material, possibly in layers, with compressive stress towards the bottom of the structure and tensile stress towards the top. The material with the variable stress resides on a release layer. When an end or ends of the structure formed of the material is freed from the release layer, the stress variant causes the ends of the structure to curl up out of the plane of the substrate upon which the release layer and material reside.
The methods of forming the variable stress structure include sputtering or electroplating as examples. U.S. Pat. No. 5,613,861, “Photolithographically Patterned Spring Contact,” issued Mar. 25, 1997 describes a method of forming the structure by sputtering. When sputter-depositing a metal, a plate of the metal, called the target, is placed on a cathode, which is set to a high negative potential and immersed in a low-pressure gas. This causes a glow-discharge plasma to ignite, from which positive ions are accelerated into the negatively charged target. This ion bombardment knocks metal atoms off the target, and many of these deposit on nearby surfaces, such as a substrate. The metal layer may be thought of as deposited in several sub-layers to a final thickness. A stress gradient is introduced into the metal layer by altering the stress inherent in each of the sub-layers, each sub-layer having a different level of inherent stress. These different stress levels can be introduced into each sublayer during sputter deposition in a variety of ways, including adding a reactive gas to the plasma, depositing the metal at an angle, and changing the pressure of the plasma gas.
Another approach is to form the release layer out of a conductive material and use it as an electrode in electroplating different layers having different stress properties to form the structures. For example, a first layer may be formed of a first layer, such as nickel electroplated using a first chemical bath and then a second layer formed from nickel electroplated using a second chemical bath. The second chemical bath produces nickel with different atomic structure as the first chemical bath resulting in a different stress characteristic for the second layer.
Electroplating may also be performed after the formation of the spring structures, for better stability or control of the properties of the spring structures. An example of such an electroplating process is discussed in U.S. Pat. No. 6,528,350, “Method for Fabricating a Metal Plated Spring Structure,” issued Mar. 4, 2003.
However the stress engineered metal structure is formed, it is selectively released from the release layer, allowing the ends of the structures to curl up away from the substrate. These curled structures are generally referred to as springs. One possible structure that can be made from these springs is an out-of-plane inductor. Out-of-plane inductors have an advantage over two-dimensional inductors in that the electromagnetic fields generated by running current through an out-of-plane inductor does not penetrate the substrate as much as those produced by two-dimensional inductors. This results in less eddy currents in the substrate and less energy loss.
In an example of a manufacturing process for out-of-plane inductors, two springs are manufactured facing each other in a mirrored fashion with latching structures on their ends. When the ends of each spring are released, they curl up towards each other, and the latching structures interlock, forming a coil. Examples of this manufacturing process are described in U.S. Pat. No. 6,621,141, “Out of Plane Microcoil with Ground Plane Structure,” issued Sep. 16, 2003, and U.S. Pat. No. 6,947,291, “Photolithographically Patterned Out-of-Plane Coil Structures and Method of Making,” issued Sep. 20, 2005.
The process of releasing and curling of the springs to cause them to latch requires very tight process controls and increases the cost of manufacture.